The present invention relates to a wire bonding apparatus for bonding, e.g., a gold wire to a pellet in an IC assembling process.
Conventionally, in wire bonding apparatuses used in assembling integrated circuits, a capillary penetrated by a gold wire is first pressed against and bonded to the bonding pad of a pellet, as a first bonding point. Then, the capillary is moved in the vertical direction (perpendicular to the pellet surface) and in the XY direction (parallel to the pellet surface) to let out the wire, so that the wire is bonded to the lead portion of a lead frame, as a second bonding point. Thereafter, the capillary is raised, and the wire is cut. The wiring of the IC is completed by repeating these processes.
In the aforementioned bonding operation, the bonding force and temperature employed when the capillary presses a gold ball at the end of the gold wire against the pellet surface are very important factors in terms of the bonding results.
The bonding methods include two systems; thermal compression bonding (the TC system) and thermal compression and supersonic bonding (the TS system). In the TC system, the pellet is preheated to about 300.degree. C. before bonding. In the TS system, the gold ball is pressed against the pellet, which is kept at 200.degree. C. or below; and, at the same time, the capillary is subjected to supersonic vibration, so that the gold ball is melted for bonding by the supersonic vibrational energy.
In the TS system, the capillary applies a given force to the gold ball; and, then, a supersonic vibration is applied to the capillary for a given time to melt the gold ball. Thus, a layer of an alloy of gold and aluminum develops on the pellet for bonding. In this case, the time during which the supersonic vibration is applied is fixed. Accordingly, if the amount of supersonic vibrational energy absorbed by the gold ball varies, due to malfunction of the bonding apparatus or surface conditions of the pellet, the shape of the gold ball after the compression bonding will vary, lowering the uniformity and reliability to the bonding portions.
FIG. 1A shows an ideal state in which a gold wire b is bonded to a pellet a. An aluminum film d deposited on an electrode portion c and a gold ball e are formed into an alloy layer by proper heating and supersonic vibration. The alloy layer is uniformly formed over the entire surface of the pressure-bonded ball e. In this case, the bonding strength estabilished between the pellet a and the gold ball e is great.
However, if the bonding force is too great, or if the supersonic vibration is applied too long, the gold ball e will be crushed and bulge out from the electrode portion c, as shown in FIG. 1B. Thus, the gold ball e will come into contact with peripheral wiring patterns (not shown), making the pellet a defective. If the supersonic vibration is applied for too short a time, on the other hand, the bonded ball e will be separated from the pellet a by slight vibration after bonding.